1. Field of the Invention
The present invention relates to a scanning probe microscope, and more particularly, to a two-phase scanning method and apparatus for obtaining information necessary to analyze physical properties of materials.
2. Background of the Invention
A scanning probe microscope (SPM) is an instrument applicable for analysis of physical properties on the surfaces of materials on a micro- and nanoscale. An SPM usually comprises a probe which has a cantilever and a tip attached to one end of the cantilever. Various modes of scanning are applicable to an SPM in accordance with the purpose of measurement. The basic modes of scanning are contact mode, non-contact mode and intermittent contact mode. The contact mode pertains to a static mode, and the non-contact mode and the intermittent contact mode are dynamic modes.
The contact mode is disclosed in U.S. Pat. No. 4,935,634 by Hansma et al. Under the contact mode, a probe tip is attached to one end of a bendable cantilever, and typically the probe tip continues to contact the surface of a sample during movement of the probe along the surface. Also, during such a movement, the vertical position of the probe relative to the surface can be controlled by a feedback so that deflection of the cantilever due to surface forces is held constant by a predetermined magnitude.
Under the non-contact mode and intermittent contact mode, a tip oscillator is attached to a XYZ translator which serves to provide positioning of the probe, and is connected with one end of the cantilever. Thus, the cantilever is oscillated at an amplitude depending on oscillation of the tip oscillator.
Typically, under the non-contact mode (developed by Martin et al., J. Applied Physics, 61(10), May 15, 1987), a cantilever with a probe tip vibrates at a small amplitude at close proximity to the surface of a sample such that the force gradient between the tip and the surface is sensed. Further, the vibration amplitude of a cantilever provides a feedback signal that allows tip-sample spacing to be held constant for profiling applications.
The intermittent contact mode (also called taping mode) is disclosed in U.S. Pat. Nos. 5,266,801 and 5,412,980 by Elings et al. Under the intermittent contact mode, a cantilever vibrates so that a probe tip contacts a sample surface in a very short interval of time. Scanning of the sample surface by the intermittent contact mode provides information on heterogeneity of micro-mechanical properties and surface force fields of the sample surface in addition to the 3-dimensional topography of the surface. Images of lateral force (O. Marti, Physica Scripta. Vol. T49, 1993), amplitude, frequency and phase shift (U.S. Pat. No. 5,519,212 by Elings et al.) can be constructed with an intermittent mode SPM.
Commercial SPM""s use a plurality of modes for scanning. In general, information scanned under the contact mode can be used for mapping topographical images and friction force images of a sample surface. Information scanned under the non-contact and intermittent contact mode can be used for mapping images on the change of amplitude, frequency and phase angle of an oscillating cantilever in addition to the topographical image of a sample surface. However, most images from conventional SPM""s are complicated which makes their analysis difficult, and only provide qualitative descriptions concerning micro-mechanical properties and surface force fields of a sample surface. Thus, quantitative estimation is not available from images provided by conventional SPM""s.
In order to overcome this problem, force spectroscopy can be used for quantitative estimation. Static force spectroscopy is applied to obtain a function concerning the change of a cantilever deflection versus a distance between a probe tip and a sample surface (hereinafter xe2x80x9ctip-sample distancexe2x80x9d). In such a case, the scanning mode is set as the static mode. For static force spectroscopy, conventional, commercial SPM""s are operated to position a probe at a predetermined height, vertical to any one point on a sample surface. As the probe gets closer to the sample surface from the initial vertical height, the cantilever is bent due to surface forces. During such a vertical downward movement, the change of the tip-sample distance and the change of the cantilever deflection are scanned. The cantilever deflection versus tip-sample distance curve is used for analysis on micro-mechanical properties, surface force fields (i.e. Van der Waals, electrostatic and capillary forces), elastic properties and contact adhesion of the sample (Burnham et. al., J. Vac. Sci. Technol., A7 (4), 1989; U.S. Pat. No. 5,193,383 by Burnham et al.).
However, in order to obtain the cantilever deflection versus the tip-sample distance curve according the above method, a one-dimensional scan must be conducted in a vertical direction (z-scan direction) with reference to any one point on a sample surface. In order to scan for a number of points on the sample surface, separate one-dimensional scans must be repeated along with new settings for scanning requirements at each point. Thus, considerable time is required to scan a plurality of positions on a sample surface.
Dynamic force spectroscopy is applied to acquire functions directed to the change of amplitude, frequency and phase angle of a cantilever with respect to the tip-sample distance. In such a case, scanning mode is set as the dynamic mode. The identification of these relationships is of great importance for characterization of surface force gradient, stiffness (elasticity) and viscoelasticity of materials, and also for the physical interpretation of amplitude, frequency and phase angle images which are required to map micro-mechanical properties of the surface or surface layer (Ducker et al., Appl. Phys. Lett. 56 (24), Jun. 11, 1990; Olsson et al., Ultramicroscopy, 42-44, 1992). However, conventional commercialized SPM""s do not provide routine procedures for scanning dynamic parameters (e.g. amplitude, frequency or phase angle) of a cantilever relative to a tip-sample distance.
Further, in order to displace scan position of a probe from one point to another in the X-Y coordinate direction on a sample surface using a conventional SPM, a piezoelectric scanner tube (which deflects when voltages are applied to electrodes thereon to produce probe movement) is bent. In such a state, a one-dimensional vertical scan is conducted. Thus, when the scan is conducted with respect to a number of points on the sample surface, the piezoelectric scanner tube is maintained in a bent position for extended periods which make it unstable. Additionally, creep is generated in the piezoelectric scanner tube, which results in unstable and inaccurate positioning of the probe tip to an aimed point. Due to such problems, conventional SPM""s cannot provide reliable results even if numerous scans are taken in a spacious area of a sample surface.
Accordingly, an object of the present invention is to overcome the above noted drawbacks and limitations of the prior art and provide an SPM and a method of operating an SPM which ensure rapid and stable scanning of a sample surface and produce reliable scan results.
To this end, the present invention provides a two-phase scanning method and apparatus. In order to obtain topographical information of a sample surface and other information such as friction force or phase angle images, the present invention initially provides a first scan at predetermined positions on the sample surface, while a probe moves along the sample surface. During the first scan, the positioning of the probe is controlled by an electronic feedback so that a deflection or amplitude of a cantilever is maintained at a predetermined setpoint. Data obtained from the first scan are stored in a memory, and an average plane of the sample surface is calculated based on the data. An imaginary line is defined on the average plane in order to specify an interested area of the sample surface in which analysis is required. An imaginary inclined plane, one side of which intersects the imaginary line and which makes a predetermined angle with reference to the average plane, is defined over the interested area. The probe is positioned at a predetermined height on the inclined plane. Then, the probe conducts a second scan at predetermined positions on the inclined plane, while it moves downward along the inclined plane until it reaches a position where deflection (in case of static mode) or amplitude (in case of dynamic mode) of the cantilever meets a predetermined setpoint. As a result of the second scan, information on deflection of the cantilever is obtained if the static mode was set as scanning mode, and information on amplitude, frequency and phase angle of the cantilever is obtained if the dynamic mode was set as scanning mode. Such information is stored in the memory. If necessary, the second scan is conducted throughout the area of interest on the sample surface. The tip-sample distances at a plurality of scan positions on the inclined plane are respectively calculated based on information provided by the first and second scans. Statistical analysis on the characteristic parameters (such as the deflection, amplitude, frequency or phase angle of the cantilever) depending on the tip-sample distance is conducted throughout the area of interest on the sample surface. Based on such analysis, a set of average parameters versus the tip-sample distance curves are obtained.
The above two-phase scanning is conducted by a scanning probe microscope of the present invention which comprises, a means for moving a probe to provide positioning of the probe with respect to a sample surface; a means for detecting change of deflection of a cantilever or change of amplitude, frequency and phase angle of the cantilever; a means for controlling the prove movement means, wherein the probe movement means is controlled based on data received from the detecting means so that deflection or amplitude of the cantilever maintains a predetermined setpoint during a first scan of the probe along the sample surface, wherein the probe is positioned at a predetermined height on an imaginary inclined plane which makes a predetermined angle with reference to an average plane of the sample surface and which is defined over the area of interest on the sample surface, and wherein the probe movement means is controlled so that a second scan is provided at a plurality of positions on the inclined plane during downward movement of the probe along the inclined plane; a means for storing data transmitted from the control means; and a means for analyzing the stored data, wherein an average plane of the sample surface is determined based on data from the first scan, and wherein the imaginary inclined plane is defined over the area of interest on the sample surface.
Other objects, features and advantages of the invention will become apparent from the following detailed description viewed in conjunction with the accompanying drawings.